Fix error checking for python json & pkg_resources

I forgot safesystem() raises an exception when it fails.

Now, if users have a python that is missing a feature we need in order
to have fpm build a package, you'll get an error like this:

      Your python environment is missing json support (either json or simplejson python module). I cannot continue without this.  {:error=>#<FPM::Util::ProcessFailed: /bin/bash failed (exit code 1).  Full command was:["/bin/bash", "-c", "/opt/my-py24/bin/python -c 'import simplejson'"]>, :python=>"/opt/my-py24/bin/python", :level=>:error}
      Process failed: Python (/opt/my-py24/bin/python) is missing simplejson or json modules. {:level=>:error}

This is for #664